1. Field of the Invention
The current invention presents a microlens array for generating a plurality of focused beamlets or focusing beams with different incident angles in a charged particle beam exposure or imaging apparatus with zero field curvature, minimized geometrical aberrations such as coma and astigmatism, comprising a current limiting aperture, generating a plurality of charged particle beamlets, a lens array aligned with the current limiting aperture, for focusing all the beamlets into a flat image plane. It solves the problem of how to generate a plurality of focussed beamlets with a minimum of aberrations in a diverging beam system, such as in multi-beamlet inspection systems, in particular desired at a limited source to target distance such as in a multi beamlet, maskless lithography system.
2. Description of Related Art
The progress in microelectronics, microfabrication and material science demand an ever-increasing spatial resolution and throughput in charged particle beam lithography and inspection. Conventional single beam systems suffer from coulomb blur and low throughput. Several charged particle beam systems featuring multi-beam, multi-column and/or multi-source are under development to solve the contradictory requirements. To increase the throughput of charged particle beam systems and avoid coulomb blur however, a large exposure field is desired, which requires the use of not only axial, but also off-axial beam of the diverging beam emitted from a charged particle emitting source. By introducing a lens array that can generate a plurality of focused beams, i.e. can focus beams with different incident angles Coulomb blur can be avoided as disclosed by Applicants earlier patent publication cited hereafter. At the same time, a tight control of the off-axial aberrations is desired.
In JP60031225, JP60039828 and J. Vac. Sci. Technol. B 4(5), September/October 1986, an electron matrix lens with reduced aberrations is disclosed where the centre of a current limiting aperture is shifted from the optical axis of a lens associated with said aperture, to an optimum position. The position of the current limiting aperture, which is fabricated in a separate plate, is chosen such that a virtual aperture, which is symmetrical along the optical axis, is in a position causing the total aberration for off-axial lenses to be minimal. Patent publication JP60042825 discloses a correction means for the field curvature for each lens, by changing the focus using a correction lens matrix. However, the astigmatism becomes dominant with increasing incident angle and eventually restricts the maximum incident angle to less than 30 mrad, thus the throughput of the system is limited. The throughput of the system is also restricted due to a small tilling factor of the lens that is allowed.
In WO2004/081910 in the name of Applicant, a lens array forming a plurality of focused beamlets from a diverging broad beam is disclosed. FIG. 10A thereof discloses a schematic of an example of the lens array, but does not indicate what the position of the electrode must be with respect to the beam. In order to reduce aberrations, the lens is, in a particular embodiment, concaved with respect to the source so that the off-axial beamlets can pass the lens along the optical axis. In this solution, the curvature of the lens plate leads to undesired engineering challenges. Also, drawbacks include that the image plane for all the beamlets is present in a concaved surface with respect to the source. Further, a difficulty exists in the alignment between the lens array and a “spatial filter”, in fact the current limiting aperture structure, which in this publication is taught to be on a planar surface.